J. Hansen, R. Ruedy, M. Sato and K. Lo
NASA Goddard Institute for Space Studies, New York, New York, USA
Abstract
We update the Goddard Institute for Space Studies (GISS) analysis of global surface temperature change. We use satellite nightlight measurements to identify measurement stations located in extreme darkness. These stations are used to adjust temperature trends of urban and peri-urban stations for non-climatic factors and to help verify that urban effects on analyzed global change are small. As the GISS analysis combines available sea surface temperature records with meteorological station measurements, we test alternative choices for the ocean record, showing that global temperature change is sensitive to estimated temperature change in polar regions where observations are limited. We compare global temperature reconstructions of GISS, NCDC, and HadCRUT. We conclude that global temperature continued to rise rapidly in the past decade, despite large year-to-year fluctuations associated with the El Nino-La Nina cycle of tropical ocean temperature.
EXCERPTS
8. Weather variability versus climate trends
Public opinion about climate change is affected by recent and ongoing weather. North America had a cool summer in 2009, perhaps the largest negative temperature anomaly on the planet (Figure 14a). Northern Hemisphere winter (Dec-Jan-Feb) of 2009-2010 was unusually cool in the United States and northern Eurasia (Figure 14b). The cool weather contributed to increased public skepticism about the concept of "global warming," especially in the United States. These regional extremes occurred despite the fact that Jun-Jul-Aug 2009 was second warmest (behind Jun-Jul-Aug 1998) and Dec-Jan-Feb 2009-2010 was second warmest (behind Dec-Jan-Feb 2006-2007).
Northern Hemisphere winter of 2009-2010 was characterized by an unusual exchange of polar and mid-latitude air. Arctic air rushed into both North America and Eurasia, and, of course, was replaced in the polar region by air from middle latitudes. Penetration of Arctic air into middle latitudes is related to the Arctic Oscillation (AO) index [Thompson & Wallace, 2000], which is defined by surface atmospheric pressure. When the AO index (Figure 15a) is positive surface pressure is low in the polar region. This helps the middle latitude jet stream blow strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar region. A negative AO index indicates relatively high pressure in the polar region, which favors weaker zonal winds, and greater movement of frigid polar air into middle latitudes.
December 2009 had the most extreme negative Arctic Oscillation since the 1970s. There were ten cases between the early 1960s and mid 1980s with negative AO index more extreme than -2.5, but no such extreme cases since then until December 2009. It is no wonder that the public had become accustomed to a reduction in the extremity of winter cold air blasts. Then, on the heels of the December 2009 anomaly, February 2010 had an even more extreme AO, the most negative AO index in the 1950-2010 period of record (Figure 15).
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Figures 9b and 10b, the 12-month running mean of global temperature, offer a more powerful way of assessing global climate change than the usual calendar-year mean temperature.
The effects of the tropical oscillations between El Ninos and La Ninas can be discerned better in the running mean graph, as well as the effect of large volcanic eruptions such as Mount Pinatubo in 1991. Furthermore, with use of the running mean it is not necessary to wait until January to see the current annual-mean global-mean surface temperature status, because the seasonal cycle is removed by the 12-month running mean just as well at any time of year.
We conclude from these data that a new record global temperature, for the period with instrumental measurements, should be set within the next few months as the effects of the recent and current moderate El Nino continue. This new record temperature will be particularly meaningful because it occurs when the recent minimum of solar irradiance (http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant) is having its maximum cooling effect.
The reality of continued global warming contrasts sharply with a frequently heard assertion that the world has been in a cooling trend for the past decade or at least "global warming stopped in 1998." Of course it is possible to find almost any trend for a limited period via judicious choice of start and end dates, but that is not a meaningful exercise. In a wiser assessment, Solomon et al. [2009] write "the trend in global surface temperature has been nearly flat since the late 1990s despite continuing increases in the forcing due to the sum of the wellmixed greenhouse gases." But is even that assertion correct?
Climate trends can be seen clearly if we take the 60-month (5-year) and 132-month (11-year) running means, as shown in Figure 21 for data through January 2010. The 5-year mean is sufficient to minimize El Nino variability, while the 11-year mean also minimizes the effect of solar variability. We conclude that there has been no reduction in the global warming trend of 0.15-0.20°C/decade that began in the late 1970s.
Link: http://data.giss.nasa.gov/gistemp/paper/gistemp2010_draft0319.pdf
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